1. Field of the Invention
The present invention relates to portable power tools, and in particular, to a portable power tool having an internal combustion engine as a driving source for operational members of the tool, in which a load applied to the engine crankshaft is unevenly distributed along the axis thereof with respect to the axis of the reciprocating motion of a piston.
2. Description of the Related Art
In conventional portable power tools, a larger component of a load may be applied to an end of the crankshaft and unevenly distributed along the axis thereof. As a result, relatively large vibrations are generated at the end of the crankshaft to which the larger component of the load is applied. Because of the large vibrations, the operator of the power tool may experience an uncomfortable sensation.
While various countermeasures against vibration have been considered in the past, none have practically and effectively reduce vibration of the power tool due to the vibration generated at the crankshaft.
Accordingly, it is an object of the present invention to provide a novel and useful portable power tool having improved vibration absorption characteristics.
To this end, the present invention provides a portable power tool comprising an internal combustion engine that includes a crankshaft operatively connected to a piston that reciprocates along a predetermined axis. A load applied to the crankshaft is unevenly distributed along the axis of the crankshaft with respect to the axis of the reciprocating movement of a piston. An operational member is driven by the internal combustion engine. A rotational body is fixed to the end of the crankshaft to which a smaller component of the load is distributed. A balancing weight is provided on the rotational body for absorbing vibration generated by the reciprocating movement of the piston at the other end of the crankshaft, i.e., the end of the crankshaft to which a larger component of the load is distributed. The balancing weight is positioned to minimize vibration. Specifically, when the piston is positioned at a top dead center, the centroid (center of gravity) of the balancing weight is disposed at an angular position that is from 140 degrees to 180 degrees (with respect to the axis of the reciprocating movement of the piston) in a rotational direction of the rotational body.
In the internal combustion engine described above, the load applied to the crankshaft is unevenly distributed along the longitudinal axis of the crankshaft with respect to the longitudinal axis of the reciprocating movement of the piston. Therefore, a relatively large vibration due to the reciprocating movement of the piston is generated at the heavily loaded end of the crankshaft. However, the vibration of the power tool is effectively absorbed since the vibration is absorbed by the balancing weight provided at the rotational body on the lightly loaded end of the crankshaft. It has been found that the vibration is absorbed or avoided most effectively by positioning the centroid of the balancing weight, when the piston is at top dead center, at an angular position of 140 degrees to 180 degrees with respect to the axis of the reciprocating movement of the piston in a rotational direction of the rotational body.
According to the present invention, the balancing weight may be an additional weight formed independently from and mounted on the rotational body. Alternatively, the balancing weight may be formed integrally with the rotational body during its manufacture.
The centroid of the balancing weight is preferably positioned substantially at 160 degrees with respect to the axis of the reciprocating movement of the piston in the rotational direction of the rotational body, when the piston is positioned at a top dead center.
The rotational body may be a ratchet-pawl holder for transmitting the rotation of a recoil-starter reel included in a recoil starter to the crankshaft. In accordance with one aspect of the present invention, the balancing weight may be a centrifugal ratchet pawl mounted on the ratchet-pawl holder. With this arrangement, it is not necessary to provide an additional component part on the rotational body, whereby the number of component parts can be reduced and an additional space for mounting a balancing weight is not required.
According to another aspect of the present invention, a portable power tool comprises an internal combustion engine that includes a crankshaft operatively connected to a piston that reciprocates along a predetermined axis. A load applied to the crankshaft is unevenly distributed along the axis of the crankshaft with respect to the axis of the reciprocating movement of a piston. An operational member is driven by the internal combustion engine. The power tool also includes a recoil starter comprising a ratchet-pawl holder fixed to an end of the crankshaft to which a smaller component of the load is distributed. A centrifugal ratchet pawl is mounted on the ratchet-pawl holder. The relational arrangement between the mass of the centrifugal ratchet pawl and the position of the centrifugal ratchet pawl relative to the piston is selected to suppress vibration generated by the reciprocating movement of the piston at an end of the crankshaft to which a larger component of the load is distributed.
It is known to provide separately prepared weights added to various parts of an internal combustion engine to reduce vibration arrangement and provide dynamic balance of the internal combustion engine during its operation. However, in accordance with this aspect of the present invention, the centrifugal ratchet pawl serves also as a balancing weight. Consequently, there is no need for an additional part that acts as a balancing weight. Moreover, since the position of the centrifugal ratchet pawl relative to the piston can be set by controlling the mounting angle of the ratchet-pawl holder with respect to the crankshaft, the balancing weight can be easily positioned.
xe2x80x9cThe mass of the ratchet pawlxe2x80x9d of the portable power tool according to this aspect of the present invention includes, other than the mass of the ratchet pawl itself, the mass of members for mounting the centrifugal ratchet pawl, such as a fixing member (e.g., screws) for mounting the ratchet pawl to the ratchet-pawl holder and an urging member (e.g., a spring) for restoring the centrifugal ratchet pawl.
The centroid of the centrifugal ratchet pawl is preferably disposed at an angular position (measured in a rotational direction of the ratchet-pawl holder) that is between 140 degrees to 180 degrees with respect to the axis of the reciprocating movement of the piston, when the piston is positioned at top dead center. In the currently preferred embodiment, the centroid of the centrifugal ratchet pawl is positioned substantially at 160 degrees with respect to the axis of the reciprocating movement of the piston in the rotational direction of the ratchet-pawl holder, when the piston is positioned at top dead center.